As a solid-state imaging device (image sensor) using a photo-electric conversion element detecting light and generating a charge, a CMOS (Complementary Metal Oxide Semiconductor) image sensor has been put into practical use. CMOS image sensors have been widely applied as parts of digital cameras, video cameras, monitoring cameras, medical endoscopes, personal computers (PC), mobile phones and other portable terminals (mobile devices), and various other electronic apparatuses.
A CMOS image sensor has photodiodes (photo-electric conversion elements) and floating diffusion (FD) amplifiers having FD layers corresponding to the individual pixels. Readout is perfumed mainly by the column parallel output method of selecting one row of the pixel array and simultaneously reading out the pixels in the column output direction.
In this regard, as the configuration of a pixel, for example, 4-transistor (4Tr) configuration pixel which has, with respect to one photodiode (photo-electric conversion element), a transfer element comprised of a transfer transistor, a reset element comprised of a reset transistor, a source-follower element comprised of a source-follower transistor, and a selection element comprised of a selection transistor can be exemplified.
The transfer transistor is selected in a predetermined transfer period and beauties a conductive state, then transfers a charge (electrons) which is generated by photo-electric conversion and accumulated (stored) in the photodiode to the floating diffusion FD. The reset transistor is selected in a predetermined reset period and becomes a conductive state, then resets the floating diffusion FD to the potential of the power supply line. The selection transistor is selected at the time of a readout scan and becomes a conductive state. Due to this, the source-follower transistor outputs, to a vertical signal line, a readout signal of column output which is obtained by converting the charge in the floating diffusion FD to a voltage signal in accordance with the charge quantity (potential).
For example, in the readout scan period, after the floating diffusion FD is reset to for example the potential of the power supply line in the reset period, the charge in the floating diffusion FD is converted to a voltage signal in accordance with the charge quantity (potential). The result is output as a readout reset voltage Vrst to the vertical signal line by the source-follower transistor. Subsequently, in the predetermined transfer period, the charge (electrons) which is generated by photo-electric conversion and accmulated in the photodiode is transferred to the floating diffusion FD. Further, the charge in the floating diffusion FD is converted to a voltage signal in accordance with the charge quantity (potential) and is output as a readout signal voltage Vsig to the vertical signal line by the source-follower transistor. The output signal of the pixel is processed as a differential signal (Vsig−Vrst).
In a CMOS image sensor having such a pixel, however, the pixel output is saturated when very strong light strikes one or more pixels, therefore there is the disadvantage that a high luminance signal is erroneously output as a low luminance signal and so-called “inversion” video noise is output.
In order to solve this, there is proposed a pixel bias circuit provided with a clipping circuit as a voltage limiter which restricts an output voltage of a pixel when very strong light strikes one pixel or more and prevents inversion video noise from being output (see for example PLT 1).